Microcapsules are small particles that contain a core material or an active material surrounded by a shell or a different matter. Depending on the size of capsule particles they are classified into different categories: e.g., nanocapsules for particles having a size of 1 μm or less, macrocapsules for 1000 μm or more, and microcapsules are those lied in between the two ranges.
Microencapsulation technique has been widely adopted for the purpose of, e.g., protecting a core material or content, shielding or concealing of an unpleasant taste or odor (drugs, foods and the like), controlling the rate of liberation to outside (perfumes, drugs and the like) and changing the phase of a core material for easy handling such as liquid core materials. In the broadest sense, microencapsulation technique is often referred to as a state where two materials are stably separated by surfactants, oil phase dispersed in a polar solvent.
Examples of core materials microencapsulated include adhesives including pressure-sensitive adhesives, agricultural chemicals, live cells, enzymes, perfumes, drugs, inks, and the like. They are intended to be used as medicines, agricultural chemicals, fragrances, pressure-sensitive copying papers and adhesives, and the like with continued release by conferring on a core material by encapsulation functions such as controlled-release, and storage stability. Microencapsulation is especially useful where it is desired to provide controlled release, stability, and safekeeping of core materials.
Particularly, demand for functional cosmetics with improved sun protection factor (SPF) is on rapid increase due to increased concern on minimizing skin damages against UV light. However, when a large amount of a UV absorbing agent is added to improve SPF values, it may cause skin troubles and deteriorate quality of cosmetics. Microcapusulation or nanocapsulation is highly demanded to give satisfaction in utility, stability and safekeeping as well as not causing any skin troubles.
Shell-forming materials encapsulating core materials are mostly organic materials such as polymers or waxes and examples of capsulation techniques, which have polymerization of monomers at the surface of two different polarities, include interfacial polymerization, in-situ polymerization, phase separation, in-liquid drying, spray drying in fluidized bed, and the like, which have been developed as drug delivery systems. Since the polymer shell materials have poor properties in chemical stability, heat resistance and strength, their usages are often limited depending on their intended purposes. It is not also preferred to control the releasing rate due to difficulty in manufacturing porous shells. The present invention uses silica, a natural inorganic material, to encapsulate a core material to improve such drawbacks associated with the use of polymer shell materials.
Many processes for preparing hollow silica capsules have been reported. F. Caruso and co-workers describe a process for burning inner polymer after silica is coated electrostatically on the surface of polymer microspheres which are dispersed into a medium [Chem. Mater., 11, 3309 (1999)]. S. Schacht et al. disclose a process for burning inner surfactant after depositing anionic silica on the surface of O/W type micelles prepared with cationic surfactant [Science, 273, 768(1996)]. Hollow silica particles are dispersed into a solvent in which an organic material is dissolved to encapsulate an organic core material using microspheres prepared by this meth7od. The silica capsules are again coated on the external surface with polymers to control the rate of release.
Further, JP Patent Publication Nos. Pyung 13-38193 and 11-29318 disclosed a method for preparing spherical silica microcapsules encapsulating a core material by employing oil-in-water system containing tetraethyl orthosilicate and an aqueous solution containing a core material and an acid. This method simultaneously encapsulates a core material as a method for encapsulating by organic polymers but requires a large amount of a surfactant to reduce the size of capsules that then causes difficulty in removing the surfactant. Further, this method is not suitable for water-soluble core materials.
Silica used as a shell material in the present invention has excellent stability at high temperature as well as against chemicals, is optically transparent in visible range, and is widely used for fillers and additives for cosmetics. Particularly, silica is advantageous in that it can be processed into a wide variety of shapes by sol-gel method using silicon alkoxide or water glass as starting materials. In general, the sol-gel technology refers to solution-based processes that undergo hydrolysis and polymerization reactions to give gels. In the sol-gel process, basic catalyst is used to enhance the process of converting sols into networked gels in microencapsulation and thus it is necessary to remove the basic catalyst that causes environmental pollution after producing microcapsules. This process may cost extra labor and expense, and further, use of a basic catalyst may be limited by the treatment regulation.
Therefore, the use of inorganic silica in micro-encapsulation process not only reduces environmental pollution by minimizing the release of a basic gelling agent in the production of microcapsules through a sol-gel reaction but also enables it to be applied in encapsulation of both hydrophilic and lipophilic core materials.